1. Field of the Invention
This invention relates generally to injection molding and more particularly to injection nozzle systems for injection molding machines.
2. Related Art
Injection nozzle systems with nozzle seals and gate inserts for insertion in the front end of a heated nozzle are well known and have various configurations. U.S. Pat. No. 4,043,740 to Gellert shows a nozzle seal which fits into a matching seat in the front end of the nozzle and has a portion which tapers inwardly around the gate. U.S. Pat. No. 4,981,431 to Schmidt discloses a nozzle seal having an outer sealing flange which is screwed into place in a seat in the front end of the heated nozzle. U.S. Pat. No. 4,875,848 to Gellert describes a gate insert which screws into place and has an integral electrical heating element. U.S. Pat. No. 5,028,227 to Gellert et al. shows a gate insert having a circumferential removal flange to permit it to be pried from the nozzle seat when removal is desired.
These nozzle systems, however, are unsatisfactory when molding materials having a narrow temperature window because heat transfer is slow along the nozzle seal and heat is lost to the surrounding cooled mold. To combat this problem, U.S. Pat. No. 5,299,928 to Gellert discloses the use of a two-piece nozzle insert, wherein an outer sealing piece is made of a material having relatively low thermal conductivity, such as titanium, and wherein an inner tip piece is made of a material having a relatively high thermal conductivity, such as beryllium copper, or a wear resistant material like tungsten carbide. This results in good heat transfer in the interior portion of the part, with an insulative effect being created by the exterior less conductive portion. However, because the inner tip piece must be made of a material such as beryllium copper or tungsten carbide, it cannot be easily and reliably threaded for attachment to the outer sealing piece of the two-piece seal. Consequently the inner tip portion is trapped in place between the seal and nozzle to hold the inner piece in place while the seal is installed in the nozzle. Typically, as shown in Gellert U.S. Pat. No. 5,299,928, this is achieved by providing the inner piece with an outwardly extending shoulder against which the outer piece can bear to securely retain the inner piece between the outer piece and the nozzle when the outer piece is threaded onto the nozzle.
A problem with conventional nozzle systems is that misalignment of the nozzle can occur due to wear or other imperfections in the threaded connection between the nozzle tip connection element and the nozzle body. It is important for the tip of injection nozzles to be aligned precisely within the gate to insure an even and unimpeded flow of melt to the melt cavities.
A problem with valve gated injection nozzles is that the valve pin that is located within the melt channel tends to become misaligned with the mold gate due to the extreme pressures exerted on the valve pin by the melt. As a result, the end of the valve pin becomes damaged over numerous cycles as it continuously engages the wall of the mold gate. The damage to the end of the pin results in imperfections in the molded parts.
Other problems associated with the molding of precision parts using valve gated injection nozzles include restricted backflow between the end of the valve pin and the mold gate, inadequate transfer of heat from the heated nozzle to the melt and inadequate change over times in cases where maintenance or color changes are required. All of the problems can contribute to flaws in the molded parts and delays in production.
Attempts have been made in the past to address these problems with valve gated injection nozzles. U.S. Pat. Nos. 4,412,807 (York), 5,254,305 (Fernandes), and 5,700,499 (Bauer) disclose various arrangements of guide surfaces defined on a valve pin and a melt channel to align the end of the valve pin within a mold gate. These devices do not adequately address backflow and thermal conductivity problems as discussed above, nor do they address the need for quick change over times to conduct maintenance or color changes. U.S. Pat. Nos. 3,716,318 (Erik) and 5,849,343 (Gellert), German Patent DE3245571 (Manner) and European Patent 638407 (Krummenacher) disclose various arrangements of guide elements having apertures for conducting the melt. A problem associated with these devices is the formation of flow lines in the molded parts due to the splitting of melt in the melt channel. The devices also suffer from the thermal conductivity and change over problems as noted with the patents described above. U.S. Pat. No. 2,865,050 (Strauss) discloses a valve gated injection nozzle for a cold runner system. The valve pin includes flattened surfaces to encourage backflow during closing of the valve pin. Strauss is not suitable for hot runner applications where freezing of the melt in the melt channel is unacceptable. Strauss of course also does not address thermal conductivity problems and also does not permit rapid change overs.
Another problem with two piece nozzle designs is that heated melt often seeps in and around the junction of the nozzle and the inner piece of the removable nozzle seal. When cooled, this resin seepage acts like a glue to stick in the nozzle seal in the nozzle end. When the connector is unthreaded in single piece devices, the “glue” is broken. However, because the inner and outer pieces of the nozzle seal are unattached in two-piece nozzles seals like that of the Gellert '928, when the outer piece is unscrewed and removed from the nozzle, the inner piece remains stuck within the nozzle. The inner piece must then be dislodged from the nozzle by other means, such as by hitting or prying the inner piece to unstick it from its seat in the nozzle end. Invariably, whatever the technique for dislodging, additional wear and/or even outright damage to the inner piece results, shortening the life of the piece.
Other multi-piece designs are also known. U.S. Pat. Nos. 5,545,028 to Hume, 5,658,604 to Gellert and 6,089,468 to Bouti show various alternatives or improvements to the design of Gellert '928, but these also suffer from the same drawback, namely that devices are still susceptible to having the tip remain stuck in the nozzle end when the seal is unscrewed and removed from the nozzle for maintenance, etc.
Also similar to the Gellert '928 configuration is the removable nozzle tip and seal insert disclosed in U.S. Pat. No. 5,208,052 to Schmidt. Here a beryllium copper tip is held in place between the nozzle and a titanium seal which is threaded to the nozzle. An insulative air space is further provided between the tip and the sleeve. A zero clearance fit exists between the tip and the sleeve in the cold condition so that, when the nozzle reaches operating temperature, the tip longitudinal growth caused by thermal expansion forces the sleeve outward and downward against the mold. While apparently providing an improved means for sealing the mold gate, the insert of Schmidt also is susceptible to remaining stuck in the nozzle end. Thus, tip damage of the type already described may still result. A further disadvantage of the Schmidt design is that the nozzle tip and sleeve require extremely accurate machining to within tight tolerances to ensure that the zero clearance sealing mechanism of the invention is effective. Such accurate machining is time-consuming and expensive.
Another removable tip and gate configuration is provided by U.S. Pat. No. 5,879,727 to Puri. Puri discloses providing an intermediate titanium or ceramic insulating element between a copper-alloy nozzle tip and a steel gate insert to thermally isolate the nozzle tip from the gate insert while permit a secure mechanically connection between the two. The tip itself joins the assembly to the nozzle end, either removably, through the provision of threads, or integrally. As described above, however, the threading of the nozzle tip is undesirable where copper-alloy tips are used and impossible if a tungsten carbide tip insert is desired. Furthermore, the additional insulating sleeve of Puri is an additional element which must be accurately machined and maintained, thereby adding to both the initial cost and the maintenance demands on the operator.
U.S. Pat. No. 4,004,871 to Hardy discloses a bi-material mold gate conduit for use in injection molding thermosetting resins. The Mold gate conduit has an inner tube welded or brazed to an outer sleeve-like body. The outer sleeve is slidably received within and pinned between co-operating mold plate members, and an annular chamber for circulating coolant around the gate is provided between the outer sleeve and the inner tube. However, because the outer sleeve is only slidably received by the assembly, there is no secure attachment provided and, further, removal can be difficult because resin leakage can freeze the conduit to the assembly, making the unit just as susceptible to damage in removal as in those devices described above.
There is a need for improved nozzle systems that overcome the above identified problems.